If a celebrity ever did join Mastodon they'd either be 1) assumed to be a parody/shitpost account, or 2) utterly mobbed because they're a celebrity and celebrities are lame.
There are still soc. media celeb wannabees if you count the people from The Other Space Where I Guess They're Important who insist on having mirror accounts here , but they never bother actually interacting with us.
🙄